Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices

Abstract

We demonstrate that the inclusion of a small amount of the co-solvent 1,8-diiodooctane in the preparation of a bulk-heterojunction photovoltaic device increases its power conversion efficiency by 20%, through a mechanism of transient plasticisation. We follow the removal of 1,8-diiodooctane directly after spin-coating using ellipsometry and ion beam analysis, while using small angle neutron scattering to characterise the morphological nanostructure evolution of the film. In PffBT4T-2OD/PC71BM devices, the power conversion efficiency increases from 7.2% to above 8.7% as a result of the coarsening of the phase domains. This coarsening process is assisted by thermal annealing and the slow evaporation of 1,8-diiodooctane, which we suggest, acts as a plasticiser to promote molecular mobility. Our results show that 1,8-diiodooctane can be completely removed from the film by a thermal annealing process at temperatures ≤100 °C and that there is an interplay between the evaporation rate of 1,8-diiodooctane and the rate of domain coarsening in the plasticized film which helps elucidate the mechanism by which additives improve device efficiency.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted N-Methyl-Fulleropyrrolidine Acceptors

“…In fact, BHJs are morphologically complex systems and, beyond the measurement of the size of phase domains, their detailed characterization would also require the measurement of the degree of purity of the phase domains and the degree of orientation (crystallinity) inside those phase domains. However, such a detailed morphological characterization would require the use of some hardly accessible techniques such as Resonant Soft X-Ray Scattering (RSoXS) [60], Small Angle Neutron Scattering (SANS) [14,61], Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) [62] and Neutron Reflectivity [63] and this is beyond the scope of our present work. At this point, we note that fullerene functionalization can affect the efficiency of OPVs in different ways.…”

“…The small band gap donor polymer PffBT4T-2OD, poly [(5,6-difluoro-2,1,3-benzothiadiazol-4,7diyl)-alt-(3,3 -di(2-octyldodecyl)-2,2 ; 5 , 2 ; 5 ,2 -quaterthio phen-5,5 "-diyl)], also commonly known as PCE11, has been attracting large interest due to its potential to fabricate high performing OPV devices [11][12][13][14][15][16]. Its high crystallinity and relatively high SCLC hole mobility (1.5-3.0 × 10 −2 cm 2 V −1 s −1 ), allow its good performance in an OPV device, when used in relatively thick (~300 nm) bulk-heterojunction (BHJ) layers with higher light absorption capabilities.…”

Different agglomeration properties of PC61BM and PC71BM in photovoltaic inks – a spin-echo SANS study

“…61 Even though these PC 61 BM and PC 71 BM concentrations are higher than those normally used in the preparation of OPV devices, this study is of particular relevance to the understanding of fullerene derivative (PC 61 BM and PC 71 BM) agglomeration, triggered by solvent evaporation, during the process of bulk-heterojunction thin lm drying. [62][63][64] In devices processed with DIO, aer active layer deposition, DIO evaporates much slower than the main solvent, 12 and this justies the relevance of our study of agglomeration also in concentrated DIO solutions. were prepared in four different solvents (chlorobenzene, toluene, chloroform and DIO) by weighting the appropriate amounts of fullerene derivatives and measuring the appropriate volumes of solvents into amber glass vials with 2 mL volume and screw cap.…”

“…[8][9][10][11] Besides the use of different processing solvents, another approach to control the morphology of BHJs and the corresponding device performance consists in adding to the photovoltaic ink solution small amounts (typically 3% v/v) of a high boiling point additive, 1,8diiodooctane (DIO) being the most popular one. [12][13][14][15] The solubility and agglomeration 16 behaviour of different fullerenes and fullerene derivatives in organic solvents and additives, with relevance for photovoltaic applications, has been the subject of signicant research in the two last decades. 17 Studies of C 60 , [18][19][20][21][22][23][24][25][26][27][28][29][30][31] C 70 , 29,[32][33][34] PC 61 BM, 2,[23][24][25][26][35][36][37][38][39][40] 14,15,38,[41][42][43] and others 2,…”

“…Fullerenes and fullerene derivatives, due to their high C/H ratio and high density, possess neutron scattering lengths very different from normal hydrogenous organic compounds and therefore no deuteration of one or more components is needed to obtain good neutron contrast between them (see Table S1 in ESI † 50 ). For this reason, neutron scattering techniques are particularly well suited for studying their agglomeration in hydrogenous organic matrices and have been previously used in the study of agglomeration in BHJ solar cells 12,51,52 and in organic solvents. 44,45,[53][54][55][56] In the particular case of neutron scattering studies of fullerenes and functionalized fullerenes in organic solvents, SANS has been used both in the Guinier and Porod regimes, namely: (i) Guinier regime in the measurement of the radius of gyration (R g ) of well dissolved individual C 60 and C 70 molecules in undersaturated solutions with no measurable fullerene-fullerene interactions [53][54][55] and (ii) Porod regime in the study of colloidal suspensions of C 60 agglomerates in CS 2 (ref.…”